- Brachial reactivity study - This ultrasound study tests how well the subject's arteries widen. The subject rests on a bed for 30 minutes. An ultrasound measuring device is placed over the artery just above the elbow. The size of the artery and blood flow through it are measured before and after inflating a pressure cuff around the forearm. The pressure cuff stops the flow of blood to the arm for a few minutes. After a 15-minute rest, the drug nitroglycerin is sprayed under the subject's tongue. Before the nitroglycerin spray and 3 minutes after it, the size of the artery and blood flow through it are measured again.